Prior art optical detectors, which include the vacuum tube type in the form of photodiodes and photomultipliers, have been widely used for a considerable length of time. In these devices the optical light signals to be detected ordinarily pass through a transparent window in the vacuum tube and are absorbed by a photoemissive cathode which provides a target for the optical energy to be detected. Electrons liberated by emission from the surface of the photoemissive cathode are accelerated by a voltage applied across the cathode and anode within the tube. In a vacuum photodiode, for example, the emitted electrons are collected by the anode directly. In a photomultiplier vacuum tube, however, the initial current emitted from the cathode is amplified by means of secondary emission of electrons by a chain of plural intermediate electrodes usually known as dynodes. In this arrangement the anode is the last electrode in the plurality of electrodes and collects the multiplied electron emission provided by the plurality of dynodes between the cathode and the anode. Sensitivity and gain of these types of devices are determined largely by the particular materials employed and also the method of preparing the cathode and the dynodes. Such vacuum tube optical detectors perform quite satisfactorily where relatively large amounts of optical energy are provided in the signals to be detected and where the light energy beams have a relatively large cross section.
Such vacuum tube optical detectors as are known in the present state of the art, however, are not well suited for fiber optic and other waveguide applications for a number of reasons. Firstly, in many fiber optic and waveguide applications a relatively small amount of optical energy is employed in the signals to be detected. Moreover, state of the art vacuum tube optical detectors are relatively large, being usually of the order of several hundred cubic centimeters in volume; in addition state of the art vacuum tube optical detectors are relatively expensive, being in a price range of approximately several hundred dollars per unit. Furthermore, they provide only a relatively low quantum efficiency in certain spectral regions; for example, providing less than 1% quantum efficiency at the 1.06 nanometer spectral range. Added to these disadvantages is the fact that such vacuum tube optical detectors typically provide a undesirably slow response of the order of approximately 2 nanoseconds rise time.
Accordingly, it is desirable that such disadvantages be eliminated wholly or in part by optical energy detection and multiplication in a photoemissive device which can be integrated with an optical waveguide in a single unit.